Q: Why is water pulled out of bays and beach fronts
as a tsunami approaches?

A: Heave a rock into a calm lake. Kersplash! Waves emanate
in widening circles from the disturbance, eventually hitting the shore. When
they reach land, what happens first? Does a wave crest fall upon the land causing
lake water to wash up the beach or does the water pull away? It depends on which
part of the wave reaches land first: the crest or the trough.

Tsunamis are no different. A cosmic body crashing into
an ocean can trigger a tsunami, so can volcanoes erupting under the sea or landslides.
Usually, however, an underwater earthquake starts the event. The seafloor buckles
where drifting plates that make up the Earth's outer shell slowly, over millions
of years, collide.

The heavier oceanic plate slides beneath the continental
plate but not always smoothly.

"While in the deepest part of the subduction zone," says
geophysicist Eric Geist, a tsunami scientist at the U.S. Geological Survey in
Menlo Park, California, " the oceanic plate creeps along as it sinks beneath
the continental plate into the mantle. At shallow depths the oceanic plate 'sticks'
to the continental plate until an earthquake occurs."

Then the oceanic plate plunges down in a sudden lurch that
snaps the continental plate like a cracked whip.

The up-thrusting and down-dropping rock moves the water
above it into a similar shape. The rock movement pushes and pulls a column of
water from the floor to the surface of the sea. The push bulges the sea above
the surface at most a few meters high; the pull drops the water into a trough.
See Figure 1. The buckled floor has moved an entire column of water into a trough-first
wave form

Waves emanate from the disturbance just as they do when
a rock hits a lake. The local tsunami travels trough-first. The wave going in
the opposite directionout to seatravels crest-first. They travel
in widening circles across the oceans of Earth for thousands of miles nearly
as fast as a jet airliner. When the waves hit the shore, they hit hard with
almost all of their initial energy.

The wave pattern changes upon reaching shallow waters near
the shore. The shape of the land-both near-shore floor and coastal contoursreflects
and refracts the waves. As a result of this jumbling, a crest of a local tsunami
may occasionally reach land before the trough.

"On the average, however," says Geist, "the trough reaches
land first." Then the waters rush out and expose the shallow seafloor. Next
the crest deluges the land with great force.

Just the opposite happens for distant tsunamis. Occasionally
a trough arrives first; then the waters recede first. Otherwise, the crest floods
the shore without warning.

Interestingly enough, when the trough hits first, the damage
may be greatest. "...both theoretical predictions and field surveys," says Frank
González, Tsunami Research Program Leader at NOAA's Pacific Marine Environmental
Laboratory in Seattle, "indicate that coastal run-up and inundation will be
greater if the trough of the leading wave precedes the crest."